A technology such as Selective Catalyst Reduction (SCR) may be utilized for NOx reduction and to achieve diesel emissions requirements. In one approach, aqueous urea is sprayed into the exhaust gas stream which subsequently reacts with NOx on the surface of an SCR catalyst, resulting in reduction of engine-out NOx emissions. For improved NOx reduction under some conditions, the liquid urea sprayed into the diesel exhaust is typically atomized and mixed before it reaches the catalyst substrate.
In one mixing approach, a two-mixer system may be utilized to provide such mixing, where a first element (e.g., an atomizer) of the system redirects the exhaust flow and catches the urea spray for atomization, and a second element (e.g., a twist mixer) aids in mixing the exhaust flow. As an example, the atomizer may include several (e.g., nine) louvers, and the twist mixer may include a helical mixing element which is welded onto a center rod.
The inventors of the present application have recognized a problem in such previous solutions. First, the atomizer is typically stamped and processed as a round element to fit into the exhaust system, and the numerous louvers add to the weight and cost of the two-mixer system. Second, the mixing element of the twist mixer typically requires separate fabrication. Further, traditional twist mixers may be welding-intensive, in that the center rod and a stiffener bar are welded at the outlet, and the whole assembly is then welded to a conical shell.
Accordingly, in one example, some of the above issues may be addressed by an exhaust passage comprising an exterior wall having an inwardly-protruding indentation traversing at least once around the exhaust passage in a helical path. The exterior wall then defines an interior passage configured to receive engine exhaust gas and direct the engine exhaust gas via the inwardly-protruding indentation. In this way, by having the indentation structure configured within the wall of the passage, separate elements traditionally welded into mixing devices may be eliminated, such as a mixing element, center rod, and stiffener bar, if desired. The exhaust passage may further include an atomizer of a partial disc shape positioned in the exhaust passage upstream of the portion comprising the inwardly-protruding indentation, which is configured to redirect exhaust flow and catch a fluid spray for atomization. The partial disc shape eliminates the overall circular design (e.g., with a circular perimeter) of traditional atomizers, and may utilize fewer louvers than traditional atomizers, thus reducing the weight and cost of the system. As such, the exhaust passage can achieve good atomization and mixing, at a lower manufacturing cost.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.